The invention relates to an electrodeless low-pressure mercury vapour discharge lamp which comprises a discharge vessel which is sealed in a gas-tight manner and contains mercury and a rare gas, which discharge vessel has a radiation-transmitting envelope and a cavity, which cavity accommodates a core of magnetic material and a wire winding surrounding said core and connected to a high-frequency supply unit, the envelope being provided with a first luminescent layer and the cavity with a second luminescent layer, two or more luminescent materials being present.
A lamp of the aforementioned kind is known from U.S. Pat. No. 4,298,828.
During operation of the electrodeless lamp the high-frequency supply unit connected to the wire winding generates a high-frequency magnetic field in the core of magnetic material, which together with the wire winding surrounding it is present inside the cavity of the discharge vessel but outside the actual discharge space. The magnetic field induces an electric field inside the discharge vessel, so that an electric discharge is maintained in this vessel. Thus short-wave ultraviolet radiation is generated, to a relatively larger degree having a wavelength of 254 nm, and to a lesser degree with a wavelength of 185 nm (mercury resonance lines). This ultraviolet radiation is converted into radiation of a greater wavelength, more particularly visible radiation, by the luminescent layer provided on the inside wall of the discharge vessel. The spectrum of the emitted radiation depends on the luminescent materials present in the luminescent layer.
Since the luminescent layer in the known electrodeless lamp not only covers the wall of the envelope, but also extends over the wall of the cavity, the luminescent material on the cavity also contributes to the conversion of short-wave ultraviolet radiation into visible radiation, which is favourable for the overall luminous efficacy of the lamp.
The U.S. Pat. No. 4,298,828 referred to further mentions that for example, the standard halophosphates can be used as luminescent materials for the luminescent layer, or that a mixture of three phosphors activated by rare earths can be used as described in U.S. Pat. No. 3,937,998.
Known low-pressure mercury vapour discharge lamps for general lighting purposes, in which the luminescent layer consists of a halophosphate with wide emission bands, for example calcium halophosphate activated by antimony and manganese, emit a substantially white light. Such lamps, however, have a moderate general colour rendering (colour rendering index R(a,8) 50-60).
The low-pressure mercury vapour discharge lamps for general lighting purposes known from the aforementioned U.S. Pat. No. 3,937,998 show emission mainly in three relatively narrow spectral regions which is why they are also called three-band fluorescent lamps. The advantage of such lamps is that they have both a good general colour rendering (colour rendering index R(a,8) of at least 80) and a high luminous efficacy (up to values of 90 lm/W and higher). This is possible since the emission of these lamps is mainly concentrated in three relatively narrow spectral bands. For this purpose the lamps contain a red luminescing material with emission mainly in the wavelength region 590-630 nm, a green luminescing material with emission mainly in the wavelength region 520-565 nm, and a blue luminescing material with emission mainly in the wavelength region 430-490 nm. The lamps emit white light of a certain colour temperature, i.e. the colour point (X, Y in the C.I.E. diagram of chromaticity coordinates) of the emitted radiation lies on or near the Planckian locus. A desired colour temperature of the light emitted by a three-band fluorescent lamp is obtained through a suitable setting of the relative contributions in the three spectral regions to the total emission of the lamp.
In the known electrodeless low-pressure mercury vapour discharge lamp provided with two or more luminescent materials, the first luminescent layer on the envelope and the second luminescent layer on the cavity are identical, i.e. they contain the same luminescent materials.
A problem in this known lamp is the lumen maintenance, which is the maintenance of the total luminous flux emitted by the lamp throughout lamp life. It has been found that the luminous flux emitted by the known lamp decreases relatively strongly during lamp life and that this, depending on the luminescent materials used, can be accompanied by an equally undesirable shift of the colour point of the radiation emitted by the lamp.